Rapid prototype wind tunnel model and method of making same

ABSTRACT

A wind tunnel model design employs a reinforcing strongback  10,  made of a rigid material, for receiving a balance  12.  At least one jacket section  14,  made of a rapid prototype (RP) material, fits over the strongback  10  and defines at least part of an aerodynamic surface. Other aerodynamic surfaces, made of either RP materials or conventional metal materials, may be attached directly to the strongback  10.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application60/246,030 filed on Nov. 6, 2000.

FIELD OF THE INVENTION

The present invention relates generally to wind tunnel models. Moreparticularly, the present invention is directed to a wind tunnel modeldesign using rapid prototype components and a reinforcing strongback.

BACKGROUND OF THE INVENTION

Whether an airframe is a new design, modification of an existing design,or evaluation of a competing or foreign design, an accurate,high-confidence representation of the airframe aerodynamics is paramountto any low-risk design or evaluation effort. These aerodynamic estimatesare used for vehicle and component sizing, performance estimates, andautopilot design and evaluation. The only accepted method of obtainingthe high fidelity aerodynamics data needed for these purposes is tobuild and test a scale model of the airframe in a wind tunnel.

Most wind tunnel models are fabricated of all metal components usingComputerized Numerical Control (CNC) milling machines. The dimensionalaccuracy, surface finish and strength of such all-metal models have adistinguished history of providing high fidelity aerodynamics data forboth subsonic and supersonic aircraft and rocket designs. However, thefabrication of all-metal wind tunnel models is very expensive and timeconsuming. Following is a brief summary of the wind tunnel modelconstruction process and of prior art attempts at reducing the costs andtime invested in such models.

A typical aircraft development program usually needs at least four tofive wind tunnel models to adequately test the aerodynamics of a newairframe. The models are generally made of aluminum (for lightlystressed components) or steel (for highly stressed components) and aresculpted using 3 to 5 axis CNC milling machines. The models can requiremonths to manufacture and are often made by small high technologycompanies that specialize in wind tunnel model manufacture.

Wind tunnel models are generally supported in a wind tunnel by apositioning device that is often referred to as a sting. The rearportion of a model is usually hollow to allow the sting to penetrate themodel body without affecting the aerodynamic properties of the model. Aforce transducer called a balance is attached to the inside of the modelin order to measure forces and moments acting on the model (oftenmeasuring all six degrees of freedom: drag, sideforce, lift, roll, pitchand yaw). The sting is rigidly fixed to the balance and all lead wiresfrom the balance and any other control lines or strain gage leads fromthe model are routed inside or along the sting and back to the controlroom of the wind tunnel facility.

The cost of fabricating and instrumenting a typical wind tunnel model ison the order of $100,000; however, complex models that include enginesimulators, remote controlled control surfaces, numerous rows ofpressure taps, etc., can cost over $1,000,000. Companies that are ableto reduce the time and costs associated with wind tunnel modelstherefore stand to gain a significant competitive advantage.

For several years Rapid Prototype (RP) materials and methods have beenconsidered as a potential source of improvements to conventional windtunnel models. RP parts can generally be made much more rapidly and lessexpensively than conventional machined parts. RP manufacturing is afield of high technology concerning the generation of three-dimensionalsolids using particles or layers of mostly polymeric materials. Two ofthe most popular RP techniques include stereolithography (SLA®) andfused deposition modeling (FDM®). Both techniques build solid objectslayer-by-layer based on data from a computer aided design (CAD) softwareprogram. SLA® equipment is manufactured by 3D Systems, Inc. of ValenciaCalif. and employs a laser beam to selectively solidify the surfacelayer of a photopolymer resin. The solidified surface layer forms across section of the prototype part. A supporting table then lowers thepart several thousandths of an inch into the resin and the lasersolidifies the next layer.

FDM® uses a proprietary technology developed by Stratasys, Inc., of EdenPrairie, Minn. It employs a movable nozzle to deposit a thread of moltenABS plastic. The thread solidifies instantly and forms the crosssectional layer of a part. A new thread is then deposited on top to formthe next layer.

Significant use of RP components in high-load wind tunnel tests has notoccurred, however, because of problems with material strength andfabrication tolerances. A study funded through the NASA Marshall SpaceFlight Center (MSFC) investigated the feasibility of using wind tunnelmodels constructed from RP materials and methods for preliminaryaerodynamic assessment of future launch vehicle configurations. See A.Springer, “Evaluating Aerodynamic Characteristics of Wind-Tunnel ModelsProduced by Rapid Prototyping Methods,” Journal of Spacecraft andRockets, Vol. 35, No. 6, November-December 1998. The study concludedthat “RP methods and materials can be used only for preliminary designstudies and limited configurations because of the RP material propertiesthat allow bending of model components under high loading conditions andthe tolerance on the fabrication processes.”

Another study was funded by Bombardier Aerospace, Inc. and conducted byMcGill University to determine whether RP techniques could replace CNCmachining of wind tunnel model components. See R. N. Chuk and V. J.Thomson, “A Comparison of Rapid Prototyping Techniques Used for WindTunnel Model Fabrication,” Rapid Prototyping Journal, Vol. 4, No. 4,1998, pp. 185-196. The study evaluated 22 different RP technologies andconcluded that “the current plastic materials of RP models do notprovide the structural integrity necessary for the survival of windtunnel models, especially for thin section parts such as tip fins andflaps.” Further, the study found that the maximum allowable dimensionsusing RP machines are generally significantly less than traditional CNCmachines and therefore larger scale single part models cannot be built.

It is clear that increased use of RP components in wind tunnel modelscould dramatically reduce the cost and time associated with wind tunnelmodel fabrication. There is therefore a need for novel wind tunnel modeldesign techniques that overcome some of the difficulties anddeficiencies involving the use of RP components.

SUMMARY OF THE INVENTION

The present invention, among other things, presents a solution to someof the aforementioned disadvantages associated with RP wind tunnelmodels.

It is therefore an object of the present invention to provide a windtunnel model design having improved strength and stiffnesscharacteristics.

Another object of the present invention is to provide a wind tunnelmodel design that is less expensive and requires significantly less timeto build than conventional designs associated with the prior art.

Yet another object of the present invention is to provide a wind tunnelmodel design that may incorporate both RP components and conventionalmetal components so as to optimize cost, construction timing, andstrength issues.

Yet another object of the present invention is to provide a wind tunnelmodel design that includes a reinforcing strongback, that may in somecases be reused, for connecting directly to a balance.

These and other objects are achieved in the present invention in a windtunnel model including a strongback, made of a rigid material and havingan exterior axial surface, the strongback being designed to be supportedby a balance. The strongback is at least partially inside the interiorvolume of a jacket section that is made of a rapid prototype (RP)material. The exterior axial surface of the strongback engages aninterior axial surface of the jacket section, and the exterior surfaceof the jacket section defines at least part of an aerodynamic surface.

The techniques of the present invention are applicable to any windtunnel model design intended for use with a balance for measuring forcesand moments applied to the model. This includes aircraft such as planes,rockets and missiles as well as ground based vehicles such as high-speedracecars.

Other objects and advantages of the invention will become more fullyapparent from the following more detailed description and the appendeddrawings that illustrate in detail one embodiment of the invention. Inthe following description, all like reference numerals refer to likeelements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a wind tunnel model of a rocketaccording to one embodiment of the present invention;

FIG. 2 is a detail view of jacket sections connected with aninterlocking tab according to the present invention;

FIG. 3 is a cross-section view of the nose section of the embodimentshown in FIG. 1;

FIG. 4 is an isometric view of several internal components of theembodiment shown in FIG. 1;

FIG. 5 is an isometric view of several of the external components of theembodiment shown in FIG. 1 with many secondary aerodynamic componentsremoved; and

FIGS. 6A and 6B are side and bottom views, respectively, of theembodiment shown in FIG. 1 with many secondary aerodynamic componentsremoved.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, the elements of the present invention includea strongback 10 around which the components of a wind tunnel model (WTM)are assembled. The strongback 10 may be connected to a balance 12 thatmeasures forces applied to the WTM. Balances 12 are typically made ofsteel and outfitted with strain-gages or other instrumentation thatmeasure forces and moments. At least one RP jacket section 14 is placedover the strongback 10 with a near-zero tolerance fit and defines atleast part of the exterior aerodynamic surface of the WTM. Secondaryaerodynamic components 16 such as a nose 26, fins 42, canards 44, inlets46, wings, engines, etc., may be attached to the RP jacket section 14or, for added strength, attached directly to the strongback 10. Wherenecessary, slots 18 (FIGS. 5, 6A and 6B) are built into the RP jacketsections 14 to allow the secondary aerodynamic components 16, e.g., themounting pad of a canard, to be placed firmly against the exteriorsurface of the strongback 10.

In the embodiment of the present invention shown in FIG. 1, the balance12 is located inside of the strongback 10. During wind tunnel testing,the balance 12 would be supported by a sting inserted through an opening40 in the rear (downwind) end of the model. However many variations ofthis design are possible. For example, the balance 12 could be onlypartially inside the strongback 10, or the balance 12 could be attachedto the rear end of a solid strongback 10.

The modular features of the present invention further allow testing ofvarious design configurations. Various secondary aerodynamic components16, and even various RP jacket sections 14, may be quickly substitutedduring testing of a single primary model design.

All RP materials used in a model according to the present invention,including RP jacket sections 14 and secondary aerodynamic components 16,may be built using any available RP technologies. These include fuseddeposition modeling, stereolithography, selective laser sintering,direct shell production casting, investment casting, solid groundcuring, and other RP technologies.

The reinforcement of the WTM provided by the strongback 10 enables thegeneration of wind tunnel test data having test-to-test repeatabilitycomparable to all-metal WTM's. The strongback 10 provides increasedstiffness to the final model and is therefore generally constructed froma high-strength material. Steel or aluminum are preferred materialsbecause they are easy to machine; however, other materials includingvarious other metal alloys and wood are also excellent candidatematerials for the strongback 10. In some embodiments of the presentinvention the strongback 10 may be reusable so as to further decreasecost and timing requirements for new wind tunnel model designs.

Larger scale models may also be manufactured using the techniques of thepresent invention because there is no need for an entire wind tunnelmodel to fit inside of an RP fabrication machine, such as an FDM® orSLA® machine. The RP jacket section(s) 14 and components of the WTM maybe fabricated separately and then assembled or attached directly to thestrongback 10.

The following non-limiting example provides a detailed description ofone embodiment of a wind tunnel model design of a rocket in accordancewith the present invention.

EXAMPLE

FIGS. 1-6 illustrate various views of a 15% scale rocket designaccording to the present invention. The rocket model was manufacturedusing FDM®-ABS plastic jacket sections 14 attached to a cylindricalsteel strongback 10. The strongback 10 provides strength and rigidity tothe plastic model and also allows larger scale models to be built. Thestrongback 10, fabricated from 304 stainless steel, is a 17.625-inchlong cylinder with a 2.25-inch outer diameter and a 1.874-inch innerdiameter. The surface of the cylinder has a surface finish of 32.

A balance adapter 20 was fabricated by the Lockheed Martin Missile andFire Control-Dallas HSWT, which also performed the final honing of thestrongback 10 inside diameter to achieve a zero tolerance fit betweenthe balance adapter 20 and the strongback 10. The inner surface of thestrongback 10 was only machined from one end to accommodate the balanceadapter 20. The remainder of the strongback 10 is unfinished. Threadedscrew holes 22 for attachment of canards 44, fins 42, and a faired inlet46 were located in the strongback 10 to meet model design requirements.

The inside forward end of the strongback 10 was machined to a 2.125 inchdiameter and threaded to accommodate a spindle 24 for attachment of themodel nose 26. The spindle 24 is also fabricated of 304 stainless steeland has a 2.125-inch diameter threaded base for mating to the strongback10 and a 0.5-inch diameter threaded spindle 24 for attachment of the ABSplastic nose 26.

The ABS plastic nose 26, fuselage jacket sections 14, faired inlet 46,fins 42, canards 44, launch lugs 38, and assorted filler blocks 28 wereall manufactured using a Stratasys Inc. FDM-1650 machine. The machinehas a 9-inch maximum limit on component length and a quotedmanufacturing tolerance of ±0.005 inch. For the present 15% scale model,this tolerance scales to ±0.030 inch on the full-scale vehicle, which iscomparable to a full-scale manufacturing specification. If the model hadbeen limited to the maximum 9-inch length—as required by many prior artRP wind tunnel model designs—the same ±0.005-inch manufacturingtolerance becomes ±0.094 inch full scale, well in excess of thefull-scale specification.

The RP parts were designed and solid geometry models were created usingPro-E design software and output as an .stl file. (This is an outputoption built into Pro-E.) The geometry information contained within the.stl file was then mathematically broken down into horizontal slices andtransferred to the FDM® machine for fabrication.

The FDM-1650 operates at high temperature to melt the ABS plastic. Thematerial is fed into a temperature-controlled extrusion head, where itis heated to a semi-liquid state. The melted plastic comes out in anextruded string of hot liquid and paints an ultra-thin layer of plastic0.010 inch thick onto a fixtureless base. The layers are built one onthe other. The material solidifies, laminating the preceding layer.Because the plastic is hot and therefore very pliable, a supportingsystem was built underneath to support the prototype pieces.

Because of FDM-1650 size constraints, the fuselage was built up as threejacket sections 14 plus the nose 26. The inner diameter of the jacketsections 14 was chosen to be equal to the outer diameter of thestrongback 10. Because of shrinkage during fabrication, the inside ofthe jacket sections 14 were lightly sanded to achieve a near-zerotolerance fit over the strongback 10. The outer surfaces of the jacketsections 14 were sanded to smooth the surface and remove the burrs thataccumulate as the part is grown. The outer surfaces were then sprayedwith an aerosol solvent (Sandfree®) and wiped clean to produce a smoothclean surface. There are no attachments between the ABS fuselage jacketsections 14 and the strongback 10.

As shown in FIG. 2, each fuselage jacket section 14 has an interlockingtab 32 to locate and attach it to the next jacket section 14. The tabs32 help hold the entire assembly together as well as improve thefidelity and roundness of the fuselage. Longitudinal and rotationalposition is maintained on the strongback 10 via attachment of fins 42,canards 44, faired inlet 46, or filler blocks 28 for these secondaryaerodynamic components 16. Slots 18 and holes 48 in the individualfuselage jacket sections 14 allow the secondary aerodynamic components16 to attach directly to the strongback 10 using the threaded screwholes 22. The slots 18 and holes 48 are incorporated into the designduring the Pro-E geometry development and included in the .stl filedescription. As such, these geometry details are created as each jacketsection 14 is grown on the FDM-1650. Screw holes to attach launch lugs38 were drilled into the center and aft fuselage jacket sections 14after fabrication on the FDM® machine. Helicoils 36, such as the oneshown in FIGS. 1 & 3 for attaching the nose, were inserted into thedrilled holes to provide a threaded surface.

The nose 26 was created in the same fashion as the fuselage jacketsections 14 with the exception that a hole was designed into a basesection of the nose 26. After fabrication, the helicoil 36 was insertedinto the hole to provide a threaded surface for attachment to thespindle 24.

The ABS launch lugs 38 and secondary aerodynamic components 16 were allfabricated in the same manner as the fuselage jacket sections 14. Thefins 42 and canards 44 were small enough to allow them to be fabricatedfour at a time in the FDM® machine.

In addition to the rapid prototype ABS fins 42 and canards 44,additional fins 42, canards 44 and other secondary aerodynamiccomponents 16 were fabricated using the following processes: (a)infusing epoxy into the ABS components; (b) casting the components outof resin using a soft mold created from a specially prepared ABScomponent; or (c) stamping the components out of sheet metal andsheathing them in ABS plastic.

(a) The epoxy-infused components were created on the FDM® machine andsurface sanded. They were then immersed in an epoxy bath and subjectedto a vacuum to remove the air contained within the rapid prototype partsand replace it with the resin epoxy. The parts were then removed fromthe container, excess resin was wiped from the outer surface of thepart, and the resin-impregnated part was allowed to cure. This resultsin a FDM® rapid prototype plastic part that is less porous and hassignificantly greater mechanical properties than that of the originalpart. The final cured part was then sanded and wiped with the Sandfree®aerosol solvent to provide the relatively smooth surface used in thewind tunnel test.

(b) The cast resin components were fabricated by first growing a masterfin and canard in the FDM® machine. These components were then sandedand treated with body filler to fill in surface pores and cracks. Next,a sandable primer paint was applied to coat the surface. The tail wasthen staged into clay to a break-away plane, using care to remove anyexcess clay from surfaces and edges. A release agent was sprayed on thetail and clay surfaces. A metal frame was constructed around the clay tocontain the pouring of silicon resin. Once cured, the silicon mold wasturned over and the other half was prepared with release agent andpoured with silicon to mold the other half of the tail along thebreak-away plane. The removal of the rapid prototype tail left aninternal cavity in the tail. With the use of some vent holes, ahypodermic needle was used to pressure fill the cavity with an epoxyresin. If high-pressure injection plastic casting is required, a hardtool mold can be made from the staged clay process by substituting ametal epoxy surface coat for the silicon. Thermalset plastics such aspolycarbonates can also be used to make parts having higher strength anddurability if required.

(c) The last process listed above for fabricating the secondaryaerodynamic components used sheet metal as a reinforcing core. HollowABS plastic sheaths were fabricated in the FDM machine. The sheathsformed the aerodynamic surfaces and fit tightly over the sheet metalcores.

After all components were fabricated, the assembly of the model on thestrongback 10 proceeded as follows: the forward fuselage jacket section14 was slid onto the strongback 10 from the aft end of the strongback 10until only an interlocking tab 32 remained; the center fuselage jacketsection 14 was mated to the forward section via an interlocking tab 32and the combined sections slid forward until only the center fuselagejacket section tab 32 remained off the strongback 10; the aftfuselage/boattail jacket section 14 was attached via an interlocking tab32 and the three combined sections translated forward to align thenecessary holes 22 in the strongback 10 to the matching slots 18 andholes 48 in the jacket sections 14; finally, the nose 26 of the modelwas attached to the strongback 10 by screwing the metal spindle 24 intothe forward end of the strongback 10 and then screwing the threaded ABSnose 26 onto the spindle 24.

At this point the entire fuselage was completely assembled and theremaining components were added as needed. For the present model,several configuration options were available, with all the optionalcomponents attached using screws and common screw holes.

If the body-alone configuration were to be tested, the filler blocks 28would be installed in lieu of the canards 44, inlet 46, and fins 42,etc. This creates a clean symmetric cone-cylinder configuration so thatthe model's symmetry characteristics may be quantified as a function ofroll angle. If the “full-up” configuration was desired, the canards 44are installed using four screws per canard. These screws pass throughthe canard mounting pad and screw directly into the strongback 10. Thefins 42 and inlet 46 attach in the same way. Because the launch lugs 38are not subjected to large aerodynamic forces, it was satisfactory toscrew them into the ABS fuselage jacket sections 14 using helicoilattachments 36.

In summary, as demonstrated in the above detailed example, the presentinvention provides for a rapid prototype wind tunnel model havingimproved strength and stiffness characteristics. Other advantages of thepresent invention include wind tunnel model designs that are lessexpensive and require significantly less time to build than conventionaldesigns associated with the prior art. Also, designs according to thepresent invention may incorporate both RP components and conventionalmetal components, e.g., RP fuselage jacket sections 14 and metal canards44, so as to optimize cost, construction timing, and strength issues.Finally, the reinforcing strongback 10 of the present invention may bereusable to further decrease costs and timing associated with a new windtunnel model design.

While the above description contains many specifics, the reader shouldnot construe these as limitations on the scope of the invention, butmerely as examples of specific embodiments thereof. Those skilled in theart will envision many other possible variations that are within itsscope. Accordingly, the reader is requested to determine the scope ofthe invention by the appended claims and their legal equivalents, andnot by the specific embodiments given above.

What is claimed is:
 1. A wind tunnel model comprising: a strongback,comprising a rigid material and having an exterior axial surface, saidstrongback designed to be supported by a balance; and at least onejacket section, made of a rapid prototype (RP) material, having aninterior axial surface and an exterior surface, said interior axialsurface of said jacket section defining an interior volume, saidstrongback being at least partially inside said interior volume of saidjacket section such that said exterior axial surface of said strongbackengages said interior axial surface of said jacket section, saidexterior surface of said jacket section defining at least part of anaerodynamic surface.
 2. The wind tunnel model of claim 1, furthercomprising a balance, said balance rigidly connected to said strongbackand said strongback rigidly connected to said jacket section, whereby,when said balance is supported by a wind tunnel facility sting, allaerodynamic forces and moments acting on said jacket section aretransferred first to said strongback and then to said balance.
 3. Thewind tunnel model of claim 1 wherein said strongback is made of amaterial selected from the group consisting of steel, aluminum, metalalloys and wood.
 4. The wind tunnel model of claim 1 wherein said jacketsection is made of an RP material selected from the group consisting ofphotopolymers, thermoplastics, cast ceramic powders, and sinteredpowdered metals.
 5. The wind tunnel model of claim 1 wherein said jacketsection is made of ABS plastic infused with resin.
 6. The wind tunnelmodel of claim 1 further comprising: a secondary aerodynamic componentconnected to said jacket section.
 7. The wind tunnel model of claim 1further comprising: a secondary aerodynamic component connected directlyto said strongback.
 8. The wind tunnel model of claim 6 or claim 7wherein said secondary aerodynamic component is a wing, fin, canard,nose, engine or inlet.
 9. The wind tunnel model of claim 6 or claim 7wherein said secondary aerodynamic component is made of a materialselected from the group consisting of steel, aluminum, titanium, metalalloys, wood, photopolymers, thermoplastics, cast ceramic powders,sintered powdered metals, and combinations thereof.
 10. The wind tunnelmodel of claim 7 wherein said jacket section defines a slot between saidexternal and internal surfaces of said jacket section for receiving saidsecondary aerodynamic component.
 11. The wind tunnel model of claim 1,further comprising a plurality of said jacket sections.
 12. The windtunnel model of claim 11, wherein said jacket sections have interlockingtabs for connecting said jacket sections together.
 13. The wind tunnelmodel of claim 1 wherein said strongback is reusable.
 14. The windtunnel model of claim 1, wherein said strongback has an interior axialsurface defining an interior volume for receiving a balance.
 15. A windtunnel model comprising: strongback means for providing a rigid,longitudinal support; jacket section means, made of a rapid prototype(RP) material, for defining an aerodynamic surface, said jacket sectionmeans fitting at least partially over said strongback means.
 16. Thewind tunnel model of claim 15 wherein said jacket section means andstrongback means are rigidly connected to a balance, whereby, when saidbalance is supported by a wind tunnel facility sting, all aerodynamicforces and moments acting on said jacket section means are transferredfirst to said strongback means and then to said balance.
 17. The windtunnel model of claim 15 wherein said strongback means is made of amaterial selected from the group consisting of steel, aluminum, metalalloys and wood.
 18. The wind tunnel model of claim 15 wherein saidjacket section means is made of an RP material selected from the groupconsisting of photopolymers, thermoplastics, cast ceramic powders, andsintered powdered metals.
 19. The wind tunnel model of claim 15 whereinsaid jacket section means is made of ABS plastic infused with resin. 20.The wind tunnel model of claim 15 further comprising: a secondaryaerodynamic component connected to said jacket section means.
 21. Thewind tunnel model of claim 15 further comprising: a secondaryaerodynamic component connected directly to said strongback means. 22.The wind tunnel model of claim 20 or claim 21 wherein said secondaryaerodynamic component is a wing, fin, canard, nose, engine or inlet. 23.The wind tunnel model of claim 20 or claim 21 wherein said secondaryaerodynamic component is made of a material selected from the groupconsisting of steel, aluminum, titanium, metal alloys, wood,photopolymers, thermoplastics, cast ceramic powders, sintered powderedmetals, and combinations thereof.
 24. The wind tunnel model of claim 21wherein said jacket section means defines a slot for receiving saidsecondary aerodynamic component.
 25. The wind tunnel model of claim 15further comprising a plurality of said jacket section means.
 26. Thewind tunnel model of claim 25, wherein said jacket section means haveinterlocking tabs for connecting said jacket section means together. 27.The wind tunnel model of claim 15 wherein said strongback is reusable.